1. Field of the Invention
The present invention is directed to filtering and classifying material with vibratory separators and shale shakers; to screens for such separators and shakers; and to such apparatuses useful for both dewatering and classification.
2. Description of Related Art
Vibrating screens have been employed for many years to separate particles in a wide array of industrial and oilfield applications. One common application of vibrating screens is in drilling operations to separate particles suspended in drilling fluids. The screens are generally flat or three-dimensional and are mounted generally horizontally on a vibrating mechanism or shaker that imparts either a rapidly reciprocating linear, elliptical and/or circular motion to the screen. Material from which particles are to be separated is poured onto a back end of the vibrating screen. The material generally flows toward the front end of the screen. Large particles are unable to move through the screen, remaining on top of the screen and moving toward the front of the screen where they are collected. The smaller particles and fluid flows through the screen and may be collected in a pan beneath the screen.
A vibrating screen may be formed from one or more layers of screening material, screen and/or wire mesh, which are generally described with reference to the diameter of the wires from which they are woven, the number wires per unit length (called a mesh count) and the shape or size of the openings between wires. The type of mesh chosen depends on the application. The screening material in certain prior art screens is mounted on a frame, a perforated plate or perforated panel.
Some screens use wire mesh that is tensioned. Tensioning (accomplished in a variety of ways, e.g. with hookstrips and/or pretensioned panels) restricts movement of the wires and assists in holding the shape of the wire mesh.
Multiple layers of mesh may be used to alleviate blinding. U.S. Pat. No. 4,033,865, describes layering two meshes in a manner that results in at least one wire of the lower of the two meshes bisecting each opening in the upper mesh. A third "backing" layer of relatively coarse, mill grade mesh is often used to carry most of the load on the screen and to increase the tensile strength of the screen.
Another problem faced in many applications is the tearing of a screen. The problem can be especially acute in heavy duty applications such as drilling and mining. A torn screen must be replaced or repaired. To facilitate repair, the screen layers are bonded to a rigid or semi-rigid support panel that has a pattern of openings, forming on the screen a plurality of cells of wire mesh. When a tear occurs in the screen, the mesh remaining within the cell in which the tear occurred is cut out and the cell is plugged. The capacity of the screen is diminished but its life is extended. Typically, several cells of a screen can be repaired before its capacity drops far enough to require replacement. Unfortunately, bonding the screen all around the edges defining openings through the support panel further restricts relative movement of the layers and the wires in each mesh layer.
Blinding and tearing of a screen reduce the capacity of the screen continually through its useful life. Although efficiency can be increased by increasing the total area the screens, the size of the screen is limited in most applications, such as on drilling rigs, especially those on offshore platforms. There has thus been generally a trade-off between capacity, longevity, repairability and resistance to blinding of the screens.
There has long been a need, recognized by the present inventors, for a screen assembly with a support which will enhance screen life and repairability with high fluid conductance and flow characteristics. There has long been a need for such a screen assembly which is durable and relatively non-blinding.